When securing a printed circuit board in a housing there is a variation, or tolerance, of the board thickness of the printed circuit board, the top and bottom housing snap features, and the top and bottom housing printed circuit board support features. These tolerances need to be compensated for to ensure that the printed circuit board is held securely, and is prevented from movement or vibration. In order to compensate for the above-referenced tolerances, crush ribs (or crumple zones), are typically used on either the top or bottom housing in an assembly not using screws.
In the case of some printed circuit board modules, there are connectors associated with the printed circuit board module that need to be located at a precise height. Because crush ribs are crushed, or compressed, upon assembly of the top and bottom housing, the crush ribs cannot be located on the housing component located on the same side of the housing assembly as the connectors because the height of the connectors will vary depending upon the degree of crushing of the crush ribs. This amount of variance would not allow for the height of the connectors to be maintained. In order to achieve a precise height of the connectors upon assembly of the printed circuit board housing, a hard strut is needed to control the height, and the crush ribs would have to be positioned on the housing component opposite that of the one located on the same side as the connectors, which in this instance would be the top housing.
In the case of some printed circuit boards, there are also temperature issues, wherein the top housing heats up. In order to dissipate this excess heat away and expel it into the air, a heat conductive paste or pad can be used to make a thermal connection with the top housing to try and draw the heat away. Because the heat conductive properties of plastic are not as good as those of metal, a plastic housing cannot be used in situations where the top housing of a printed circuit board heats up beyond an acceptable amount. In these situations, a metal housing cover is preferred to better dissipate the excess heat.
However, if the connectors are associated with the bottom housing component and the opposing top housing component is made of metal, the crush ribs could not be located on the bottom housing or the top metal housing because the height of the connectors could vary upon assembly if the crush ribs were located on the bottom housing as described above, and the crush ribs would not “crush” and only dig into the printed circuit board if they were located on the top metal housing.
Accordingly it is desirable for crush ribs to be located on the side of the printed circuit board assembly having a top portion that is metal, wherein the crush ribs are still “crushable.”